Actress Margot Kidder, best known for her role as Lois Lane in “Superman,” has died aged 69.

A funeral home in Livingston, Montana, where the actress lived, said Kidder died at her home Sunday.

She rose to fame starring alongside Christopher Reeve in the “Superman” films of the 1970s and 1980s.

The Canadian-born actress acquired American citizenship in 2005, and became a political and women’s rights activist alongside her acting.

The cause of her death is not yet known.

Kidder starred alongside Reeve in the 1978 film “Superman” and its sequels, as well as horror classics “Black Christmas” and “The Amityville Horror.”

The actress was also an outspoken critic of the “Gulf War,” of fracking by energy companies, and was at times a vocal supporter of Democratic party candidates.

After settling in the U.S. state of Montana, she became a supporter of Montana Women For, a non-profit organization which describes its goals as the “participation and empowerment of women in our democracy through education and advocacy* on critical issues.”

As an activist, she was arrested in 2011 while taking part in a protest at the White House against the Keystone XL pipeline, which remains controversial* today.

Kidder also suffered from mental health problems, which resulted in her high-profile disappearance for several days in 1996.

In an interview with People magazine later that year, she referred to her disappearance as “the most public freak-out* in history.”